Balcony Junction Psi Value Calculator (E8 / E9 / E23)

E8 / E9 / E23 SAP / Approved Document L reference junction

Schematic cross-section of the balcony junction (E8 / E9 / E23) showing the structure, insulation line and the heat-bridge path — Schematic of the balcony junction (E8 / E9 / E23) — not to scale. Model your actual build-up in ΨMonkey.

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What the balcony junction is

The balcony junction is where a balcony or canopy structure connects to the building — often a concrete slab or steel members cantilevering out through the external wall.

Why it matters

A cantilevered slab or steel that passes straight through the insulated envelope is one of the strongest thermal bridges in any building, carrying heat directly outside along its full length unless a structural thermal break is used.

How to model it

Model the floor slab or steel, the wall and insulation it penetrates, and any structural thermal break, then solve for the Ψ-value and the f_Rsi at the internal corner.

Without a thermal break the Ψ-value is high; a proprietary structural break dramatically reduces it — and the calculation shows the difference.

What ΨMonkey solves this junction at

ΨMonkey's own results for its standard balcony junction templates, with the external wall set to each of the assistant's eight named build-ups and everything else left at the template default. They are what the tool gives for those exact layers — not accredited values, and not your detail: a different block, a thicker board or a different lintel plate all move the number. Open any figure and the app loads that junction and wall with Ψ live; change what differs and read your own.

Junction	Masonry, full fillU 0.18	Masonry, partial fillU 0.28	Timber frame, brickU 0.27	TF brick + service voidU 0.26	TF + 50 PIR sheathingU 0.16	Timber frame, renderU 0.28	Twin stud, 300 celluloseU 0.11	CLT + EWIU 0.18
E8Balcony — insulation continuousi	0.0000.977	0.0440.959	−0.0030.965	0.0000.966	0.0160.977	−0.0030.964	−0.0010.986	0.0010.977
E9Balcony between dwellings	0.0140.977	0.0660.959	0.0180.965	0.0210.966	0.0290.977	0.0190.964	0.0080.986	0.0160.977
E23Balcony — support penetrates insulation	0.2690.791	0.2960.771	0.2290.757	0.2250.777	0.2310.803	0.2510.740	0.1720.848	0.2340.813

Ψ in W/(m·K), with f_Rsi beneath. Red is f_Rsi below 0.75, the BRE IP 1/06 condensation criterion for dwellings. Marked negatives on a non-inverted junction mean the template's default detail doesn't suit that wall — a prompt to model your own, not a figure to use. Click any figure to open that junction and wall in ΨMonkey with Ψ live.

The eight walls, and how these figures were produced

Cavity masonry — full fill (accredited default)U 0.176 W/m²K100 mm Facing brick · 100 mm PIR insulation (foil-faced) · 100 mm Aerated concrete block (general) · 20 mm Plasterboard on dabs (12.5 + 15mm)
Cavity masonry — partial fill + 50 mm clear cavityU 0.278 W/m²K100 mm Facing brick · 50 mm Air cavity (ISO 6946, ≥25mm) · 50 mm PIR insulation (foil-faced) · 100 mm Aerated concrete block (general) · 20 mm Plasterboard on dabs (12.5 + 15mm)
Timber frame — brick clad, 140 mm studs (15% bridged)U 0.271 W/m²K100 mm Facing brick · 50 mm Air cavity (ISO 6946, ≥25mm) · 20 mm OSB/3 (18mm) · 140 mm Mineral wool (glass/rock) + 15% Structural timber · 20 mm Plasterboard on dabs (12.5 + 15mm)
Timber frame — brick clad, 140 studs + service voidU 0.258 W/m²K100 mm Facing brick · 50 mm Air cavity (ISO 6946, ≥25mm) · 20 mm OSB/3 (18mm) · 140 mm Mineral wool (glass/rock) + 15% Structural timber · 25 mm Air cavity (ISO 6946, ≥25mm) + 9% Structural timber · 20 mm Plasterboard on dabs (12.5 + 15mm)
Timber frame — 140 studs + 50 mm PIR over the sheathingU 0.163 W/m²K100 mm Facing brick · 50 mm Air cavity (ISO 6946, ≥25mm) · 50 mm PIR insulation (foil-faced) · 20 mm OSB/3 (18mm) · 140 mm Mineral wool (glass/rock) + 15% Structural timber · 25 mm Air cavity (ISO 6946, ≥25mm) + 9% Structural timber · 20 mm Plasterboard on dabs (12.5 + 15mm)
Timber frame — render / rainscreen, 140 mm studsU 0.278 W/m²K20 mm External silicone render · 50 mm Air cavity (ISO 6946, ≥25mm) · 20 mm OSB/3 (18mm) · 140 mm Mineral wool (glass/rock) + 15% Structural timber · 20 mm Plasterboard on dabs (12.5 + 15mm)
Closed panel / twin stud — 300 mm cellulose (8% bridged)U 0.112 W/m²K20 mm External silicone render · 50 mm Air cavity (ISO 6946, ≥25mm) · 60 mm Woodfibre board (Steico) · 300 mm Cellulose fibre (blown) + 8% Structural timber · 20 mm OSB/3 (18mm) · 20 mm Plasterboard on dabs (12.5 + 15mm)
CLT + external insulation — 100 CLT, 160 woodfibreU 0.179 W/m²K20 mm External silicone render · 160 mm Woodfibre board (Steico) · 100 mm Cross-laminated timber (CLT) · 25 mm Air cavity (ISO 6946, ≥25mm) + 9% Structural timber · 20 mm Plasterboard on dabs (12.5 + 15mm)

Each figure is ΨMonkey's own result for its standard template of that junction, with the external wall set to the named build-up and every other element — floors, roofs, party walls, lintels, windows — at the template's shipped default. Boundary conditions to BS EN ISO 10211, Ti 20 °C / Te 0 °C, convergence 10⁻⁶. Solved 2 September 2026 on the app as shipped; regenerated whenever the app changes. How the solver itself is checked is on the validation page.

The full set on one page: the Ψ-value reference table.

Related build-ups: the U-values either side of this junction

The Ψ-value at the balcony junction depends on the walls, floors and roof around it — so the first step is knowing the U-values of those elements. Typical figures for the build-ups that pair with this junction:

Model your exact build-up and read its U-value with the free U-Monkey calculator (PDFs included free), then come back here and solve the junction in ΨMonkey.

Free to model — no subscription

ΨMonkey isn't a monthly licence or a per-seat subscription. Build the balcony junction on the grid, solve it, and read the Ψ-value and f_Rsi on screen for nothing. You only pay £35 +VAT (£42 inc VAT), once, when you need the Building Control-ready PDF for that junction — so calculating a single detail never means signing up to anything. Calculated psi values are often the cheapest route to a SAP 10 pass, so the maths can pay for itself.

Frequently asked questions

What is the balcony junction (E8 / E9 / E23)?

The balcony junction is where a balcony or canopy structure connects to the building — often a concrete slab or steel members cantilevering out through the external wall. A cantilevered slab or steel that passes straight through the insulated envelope is one of the strongest thermal bridges in any building, carrying heat directly outside along its full length unless a structural thermal break is used.

How do I calculate the balcony junction psi value?

Model the floor slab or steel, the wall and insulation it penetrates, and any structural thermal break, then solve for the Ψ-value and the f_Rsi at the internal corner. ΨMonkey solves it to BS EN ISO 10211 and returns the Ψ-value (W/m·K) and the f_Rsi temperature factor on screen, free.

Is it free, and is there a subscription?

No subscription and no licence — modelling the junction and reading off the Ψ-value is free in your browser. You only pay £35 +VAT (£42 inc VAT) when you export the Building Control-ready PDF for that junction.

What psi value should I expect for the balcony junction?

It depends on the wall. ΨMonkey's standard balcony junction templates solve between −0.003 and 0.296 W/(m·K) across ΨMonkey's 8 standard walls, with everything else at the template defaults; on the accredited full-fill masonry wall that is E8 0.000, E9 0.014, E23 0.269 W/(m·K). Those are the tool's own results for those exact layers, not accredited values — open the junction with your wall preset from the table on this page and change what differs.

Can I use the result in a SAP or HEM assessment?

Yes — a calculated Ψ-value to ISO 10211 can replace the SAP default for this junction, and Without a thermal break the Ψ-value is high; a proprietary structural break dramatically reduces it — and the calculation shows the difference.

Other junctions

ΨMonkey models any junction. Jump to another common one:

Balcony Junction Psi Value Calculator